Let's start making our terrarium by putting in the soil and other base materials. How you do this part of the terrarium is actually quite important. If you want your terrarium to be healthy and last a long time with minimal maintenance this is how I recommend you put it your layers of materials: (From the bottom up)

1. Put theUnderground Den right into the empty terrarium with the open part against the front glass
2. Put about a 1 inch layer of sand in the whole bottom of the terrarium. This wil catch and hold any excess water. I recommend you use gardening sand and not beach sand. Beach sand usually has a very high salt level or a lot of microbes and these things can be harmful to your plants because they will leech into the soil
3. Put about a 1 inch layer of stones and pebbles in the whole bottom of the tank right on top of the sand. This will insure nice drainage so your soil doesnt get too soggy. I don't recommend you use any kind limestone pebbles because the limestone can affect the PH of your terrarium and be harmful to your plants.
4. Put a 1 inch layer of spaghnum or spanish moss right on top of the pebble layer. What this does is act almost like a blanket and keep nice separation between the soil and the pebbles. This way the soil doesn't run down and mix with the pebbles and sand causing a muddy mess. It is almost like a filter.
5. Finally put about 2-3 inches of soil in the terrarium. It should go approximately to the top of your underground den.

Okay lets do it now. Follow along with the process and there are some considerations we have to make for the placement of the waterfall water pool. Here are pictures of the process:

Steps 1 and 2: The underground lair is in place, we have placed a 1 inch layer of sand on the bottom and then a 1 inch layer of stones on top of the sand.

Put the Spanish/Spaghnum Moss right on top of the stones. This should form a nice blanket covering all the stones in the terrarium. The picture shows about half way done with the moss covering. This acts as a nice insulation between soil and stones so the soil doesnt run all down to the bottom. This next picture shows a bag of spanish moss. You can purchase it at any home improvement or gardening shop and it is very cheap. Get enough to make a whole layer in your terrarium.

Let's put the soil and the preparations for the waterfall